Sand casting means injecting a molten metal into the mold space manufactured by using sands (or molding sands) and then solidifying the molten metal to obtain the product of a desirous shape.
During sand casting, a binder is used for binding molding sands. Conventional organic binders such as phenol, isocyanate, ester, and furan resins have the advantage of a rapid hardening rate. However reclamation of used molding sands is hard and such organic binders may produce odors and poisonous gases and the like, thus having the demerits causing adverse effect on the molding environment and the environmental problem.
Accordingly, the research is being carried out to use inorganic binders. In particular, the research as to inorganic binders which include alkali silicate as a main ingredient is actively being carried out. Inorganic binders including alkali silicate as the main ingredient cause a small amount of odors and gases, and thus have the advantages of being capable of preserving hygiene and safety of workers and the environment and of being capable of reusing molding sands while such inorganic binders have the problem that due to their peculiar hygroscopicity binding force weakens and strength reduces rapidly.
U.S. Pat. No. 7,712,516 discloses that use of alkali silicate and amorphous silicon oxide increases flowability of core sand without reduction of strength, thus enhancing the performance of a core but is hard to completely supplement the reduction of strength due to moisture of amorphous silicon oxide.
In addition, Korean Patent No. 10-1199111 discloses a core material mixture for casting comprising alkali silicate binder and oxide film inhibitor and a method of preventing hygroscopicity of alkali silicate due to oxide film inhibitor. However the above invention has the problem of being hard to control hygroscopicity when a coating such as a complete film is not created.